Multiplex transmission



April 12, 1927.

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Patented Apr. 12, 1927.

UNITED'T'STATES PATENT oFFicEQ.

-J ACO B'SUTER JAMMER, OF CUMBERLAND, MARYLAND, ASSIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

M'ULTIIPLEX rnansmrssroxf Application filed December 17, 1924. Serial No. 756,399.

This invention relates to wave transmis-= sion systems and is particularly adapted for use in attenuation compensating or equal-' izing circuits such as are frequently employed in multiplex carrier current signaling systems.

An object of the invention is to provide for controlling the transmission characteristic of a transmission line or system in an economical and efiicient manner.

A related object of the invention is to compensate for the unequal attenuation of the different waves in the transmission medium of a multiplex signaling system.

Another object of the invention is to reduce the requirements on the grouping filters employed at repeater points -or"els'ewhere in The invention is adapted for use in multiplex carrier current telephone and telegraph systems operated. on agroup frequency basis, wherein the different channel frequencies for transmission in one direction are grouped in a different frequency range from that employed for transmission in the opposite direction.

In multiplex systems of this type, due to the inherent attenuation of the transmiss'ion'lineand its associated ap aratus, it is frequently neces ary to amplify the transmitted waves. This may be accomplished by inserting two-way repeaters at intervals along the transmission line, so arranged that all of the waves transmitted in one direction will be amplified in one branch of the repeater andallthe waves transmitted in the opposite direction will be amplified in another branch of the repeater.

In such systems, particularly those employing long line conductors, it is well known that the waves of higher frequencies are less efficiently transmitted than those of lower frequencies. The wider the range of frequencies transmitted, the greater is the r I u I distortion due to the poorer transmission or greater attenuation of the high frequencies relative-to the low.

If a'repeater of the above type is used for a wide range of frequencies on a line possessing unequal attenuating properties, it is desirable to associate with the repeater some means for equalizing or compensating for the distortion due to such attenuation. This is desirable in order that the repeater sirable to introduce the means of correction for the l ne attenuation at the repeater, since the correcting means introduces a loss which may be made up by the repeater.

Heretofore, to meet these requirements, it has beencustomary to so design the repeater grouping filters as to ofier a minimum attenuation to the frequencies included within the transmission range of the filters," and then, in order to equalize the load on the input of the repeater "amplifier, an equalizing network had been inserted between the input filter and the amplifier. A system of this type is shown in U. S. Patent No. 1,413,357 to P. A. Raibourn, issued April 18, 1922.

When for economic reasons it becomes necessary to secure a maXimum number of' communication channels by reducing the frequency spacing between the oppositely directed groups of frequencies, repeaters of the above type are expensive to construct.

This is due not only to the cost of-the sepaof construction, thus doing away in some instances, at least, with the necessity for separate attenuation equalizing networks,

and-at the same time reducing the attenuat'ion requirements on the grouping filters.

The. invention will be described as applied to a two-way amplifying repeater for use in multiplex carrier current telephone or,

telegraph systems, but it is to be understood that it may also be applied to other places in the system than at, repeater points, and to various types of systems other than multiplex carrier systems.

The various features and objects of the invention will appear from the following description and the accompanying drawing, in which:

Fig. 1 is a diagrammatic illustration of a two-way amplifying repeater constructed in accordance with the invention.

Fig. 2 shows curves illustrating the principle of o eration of the repeater of Fig. 1.

Fig. 3 illustrates the type of high pass filter which may be employed in the system of Fig. 1 in carrying out the invention.

Fig. 4 shows curves illustrating the characteristics of the filter shown in Fi 8 depending upon the time constant of the coils employed therein.

The two-way repeater of Fig. 1 comprises two one-way repeating channels RW and RE, ada ted to connect transmission line sections and E.

The currents incoming at the repeater from line section E are amplified in the channel RE and the am lified currents are delivered to line section V. Similarly, the currents incoming from line section W are amplified in the channel RW and the amplified currents are delivered to hue section E.

The currents incoming from the line section WV are of higher frequencies than those incoming from line section E. The respective repeater channels are provided wlth frequency discriminating filters which direct the incoming currents to the proper channels.

The repeater channel RVV includes a high 'pass input filter HPIF, an amplifying structure WA and a high pass output filter HPOF. The repeater channel RE includes a low pass input filter LPIF, an amplifying structure EA and a low pass output filter LPOF.

The amplifying structures VA and EA may be of any well-known type such, for example, as that disclosed in the Raihourn patent, supra.

The directional filters HPIF and HPOF are designed to transmit currents of the upper group of frequencies to be repeated by the amplifying structure WA and to suppress from transmission currents of lower fre uencies, while the directional filters LlPI and LPOF are designed to transmit currents'of the lower group of frequencies to be repeated by the amplifying structure EA and to suppress currents of higher frequencies.

These filters may be of the general type disclosed in United States patent to G. A. Campbell, No. 1,493,600, dated May 13, 1924, and may be of the particular composite wave filter type discussed in Part III of an article on the Theory and design of uniform and composite electric wave-filters by Otto J. Zobel, in the Bell System Technical Journal of January, 1923. These filters consist in general of a plurality of sections having series and shunt reactances.

designed, according to well known laws, for the range of frequencies of the currents which the filters are to transm1t.

Since the attenuation of a transmission line is a function of the frequency, the carrier currents incoming at the repeater will vary in magnitude corresponding to the attenuation of the line which they have traversed. It is likewise evident that when these currents are passed through the repeater channels RVV or RE, as the case may he, to the succeeding line, the variations in magnitude in the second line will be proportionally much greater than; for the first line unless means are included in the repeater channels for the urposc of effectually decreasing the amplification of the amplifying structures WA and EA tor the lower or less attenuated frequencies.

According to the invention, both functions of discriminating against the frequencies which it is desired to suppress from transmission and of furnishing the equalization required for the different carrier "frequencies which the respective filters are designed to transmit are performed in certain casesby one and the same network thus dispensing with the second network heretofore thought necessary, together with its high cost of construction.

This is accomplished by a novel mode of construction in the high pass filter. llhe invention is particularly applicable to the construction of high pass filters HPlIlF and HPOF since it happens that the gradual cut-oil obtained by the cheaper design in accordance with the invention is in the right direction in the case of the higher grouping filters to aid in the attenuation equalization. The expense of such a repeater may thus be materially reduced by constructing the high pass filters in accordance with the invention. The low pass filters LPI'F and LPOF, may be constructed as heretofore in the art.

Fig. 2 shows graphically the transmission requirements to be met at a repeater point on a typical carrier line in order to compensate for the unequal line attenuation over the two groups of frequencies. In this fig ure the transmission loss is plotted against. frequency.

The curve ac of Fig. 2 represents the desired attenuation characteristic for the high pass directional filter such as has heretofore been employed to transmit with substantially uniform small attenuation all frequencies comprised within the upper group of frequencies and to greatly attenuate all frequencies below a definite limit. Similariv. the curve ac' of Fig. 2 represents the desired attenuation characteristic for the low pass directional filter designed to transmit with substantially negligible attenuation all frequencies comprised within the lower group of frequencies and to greatly attenuate all frequencies above a definite limit.

High pass filters having a characteristic actions of the American Institute of Elec trical Engineers, vol. 40, 1921.

In order to secure characteristics b'-0' and 6-0 for the circuits at the. repeater points to equalize the over-all attenuation of the line and the repeater circuits, it has been the practice to insert in circuit with the grouping filters attenuation equalizer networks wliich gave a gradual discrimination against the lower frequencies and in favor of the higher throughout the range of each grouping filter.

In the case of the high-pass filt'er this meant that the filter was designed to give a sharp cut-01f, which was to a great extentnullified by the equalizer network since the latter caused the total characteristic to assume a less abrupt form (bc) than the filter characteristic (a-0) alone. The realization of this effect led to the discovery that the characteristic b-c could be directly realized to a good approximation by a proper design of the high-pass filter alone, thus dispensing with the extra net-work heretofore employed for giving varlable attenuatlon throughout the higher-group. Not

only is this extra network dispensed with but it happens that'the high-passfilter itself designed in accordance with the invention can be more cheaply constructed than the high-pass filter heretofore employed.

Fig. 3 shows the schematic of the high pass filter of known type which is suitable for the high-pass filter as heretofore used and by proper design of its individual parts is also suitable for use in practising the invention. The curves at and e of Fig. 4 show typical (computed)- characteristics of this filter when different values of are used for the inductanccs, f being frequency, L inductance and It resistance. Curve d is the characteristic of the type suitable for use in systems of the prior art for approximating the required characteristic ac of Fig. 2. For this curve ((6) a value of Q=150 was used, which corresponds to a very low value of resistance and means that large sized copper wire would have to be used in winding the inductances, that the iron losses must be kept low and that in general, a careful and expensive design and construction must be adopted. Curve e was obtained by using a value of (1:10. This curve, it will be observed, approximates the required curve b-c directly, thus giving both groupselectivity and attenuation equalization in the same network. Further a Q-value of 10 is so low that amuch cheaper construction of filter can be used than for the curve d filter, as for example much higher resistance wire which means less copper for the same inductance.

Curve (Z was computed for a cut-off frequency of 14,500. cycles per second. while curve e was computed for a cut-off of 14,000 cycles per second.

The elements herein shown and described as well as the numerical values that have been given should be considered as merely illustrative of the invention, but are not to be construed as limiting the invention which, of course, is susceptible of various other modifications and adaptations.

lVhat is claimed is:

' 1.In a wave transmission systemin which the waves of higher frequencies in traversing the system are attenuated to a greater extent than those of lower frequencies, a selective network of a high-pass filter type having its inductances constructed with a relatively small value of {to give the proportioned to produce a gradual variation in attenuation throughout the frequency range of said channels of such amount as to compensate for the unequal line attenuations at the various channel frequencies.

In witness whereof, I hereunto subscribe having the ratio of of its inductanees my name this 11th day of December, A. D.

JACOB SUTER JAMMER. 

